I looked at the docs and mailing lists, and as far as I can see JDK 1.3 is the main target version for SLF4J. Nevertheles, I didn't find any statement for SLF4J-EXT: http://slf4j.org/faq.html#requirements

Plain proof of concept:

C:\java\libs\logging\slf4j\slf4j-1.5.11>java -version
java version "1.4.2_18"
[...]

C:\java\libs\logging\slf4j\slf4j-1.5.11>java -cp slf4j-ext-1.5.11.jar org.slf4j.profiler.Profiler
Exception in thread "main" java.lang.UnsupportedClassVersionError: org/slf4j/profiler/Profiler (Unsupported major.minor version 49.0)
[...]